As a passive optical device which branches and combines light, an optical coupler (optical fiber coupler) is used. An optical coupler is utilized not only for fiber-optic communication, but also in a confocal endoscope system, etc., in recent years. For example, as a confocal endoscope system which uses excitation light, a configuration using a 2×2 WDM (wavelength division multiplexing) type optical coupler is provided with a first port and second port on the input side (output side) and a third port and fourth port on the output side (input side) is known (PLT 1). In this confocal endoscope system, the excitation light, which is emitted from the laser light source, enters the first port and is emitted from the third port. The fluorescence of the subject due to the emitted excitation light enters the third port and is guided through the second port to a light-receiving section. Further, this optical coupler ideally is designed so that excitation light with a wavelength of 488 nm enters from the first port and branches out to the third port and fourth port at a 90:10 ratio, while fluorescent light with a peak wavelength of 515 nm enters from the third port and branches out to the first port and second port at a 0:100 ratio.